FRANCE: Mona Lisa's secret smile unveiled

The Mona Lisa has been stripped bare to reveal her most tantalising secret. Two French researchers have used virtual science to discover for the first time the technique Leonardo da Vinci used to paint the flesh tints of the lady with the enigmatic smile.

The mystery of the sfumato technique was unveiled by optical engineer Pascal Cotte, technical director and founder of Paris-based start-up company Lumière Technology, and Mady Elias of INSP, the Institute of Nanosciences of Paris, joint laboratory of the National Centre of Scientific Research and the Universities of Paris-6, Paris-7 and Evry-Val-Essonne. Their research is published in Applied Optics*, journal of the Optical Society of America.

Sfumato, from the Italian sfumare meaning to 'tone down' or 'evaporate like smoke', is the term used for fine shading in painting to produce subtle, imperceptible changes in colour and tone. It is usually associated with Leonardo da Vinci and his school, but originated with early Flemish artists such as Van Eyck and Van der Weyden.

Cotte explained sfumato was used "to blur the edges and outlines. Leonardo considered that to reproduce reality in painting there must be no obvious lines, so there had to be a kind of gradation".

The research involved examination of the 500-year-old Mona Lisa, which hangs in the Louvre museum in Paris, by Cotte's multi-spectral, 240-megapixel camera which recorded 100 million reflectance spectra on the picture, photographing the layers of paint beneath the yellowing varnish.

The data were then sent to the INSP for analysis. "We discovered the technique used, which was originated by the Flemish primitives," Elias told France's TV3. "That's to say, it's a glaze where the painter doesn't mix black or white pigments to work on the brightness, but uses only one pigment and applies a certain number of layers depending on how bright he wants it."

The analysis identified umber as the pigment used. Cotte's camera has uncovered or confirmed other facts about La Giaconda. Last year, he said multispectral digitisation had revealed "with amazing precision" that she was originally painted with eyelashes, eyebrows and a wider smile, and with a fur-lined coat on her knees. His previous researches have disclosed the 'true colours' of the painting and other works of art.

*M. Elias and P. Cotte, Multispectral camera and radiative transfer equation used to depict Leonardo's sfumato in Mona Lisa, in Applied Optics, Vol. 47, issue 12, pp 2146-2154.

This was a most exquisite discovery, explaining in part the mystique surrounding Mona Lisa. As an art writer and promoter it has assisted me to explain the work of Australian-based artist Belinda Herford, a contemporory abstract artist. This story has spread rapidly around the world and received substantial publicity in Australia. Thank you.

Anna Day